Inquiry Regarding Wind Stress Calculation in bulk_flux.F

[chenhj]

Dear ROMS Development Team,

First, I would like to express my gratitude for your continued efforts in improving the Regional Ocean Modeling System (ROMS). The recent updates, including the implementation of the COARE3.5 bulk flux algorithm and the correction of the wind stress bug (viewtopic.php?t=6502), are greatly appreciated.

While testing version 4.2, I noticed a change in the calculation of the horizontal momentum flux due to rain impact, which was not explicitly mentioned in the update notes. For clarity, I have compared the relevant code snippets below (I only show the part relating to rain impact):

In version 4.1, the part relating to rain impact is 0.85*rain(i,j)*Wmag(i)

Code: Select all

Taur=0.85_r8*rain(i,j)*Wmag(i)
Taux(i,j)=...+Taur*SIGN(1.0_r8,Uair(i,j)))
Tauy(i,j)=...+Taur*SIGN(1.0_r8,Vair(i,j)))

In version 4.2, the part relating to rain impact is 0.85*rain(i,j)*Uair(i,j) (or Vair(i,j))

Code: Select all

Taur=0.85_r8*ABS(rain(i,j))*Wmag(i)
cff=rhoAir(i)*(...+Taur/rhoAir(i))/(Wmag(i)+eps)
Taux(i,j)=cff*Uair(i,j)

Could you kindly clarify whether this modification was intentional and confirm which version aligns with the intended physics of rain-induced momentum flux?

Any guidance on this matter would be greatly appreciated, as it will help ensure the consistency of my simulations. Thank you again for your dedication to advancing ROMS and for your time in addressing this inquiry.

Hongjing

[wilkin]

Previously, taur due to rain was added as distinct vector components to taux,tauy near the end of the calculation. But in the new formulation we add the magnitude of the momentum of the rain to the magnitude of the wind stress, and then break it out into vector components in one step. This is correct provided the stress is in the same direction of the vector wind, which everyone assumes. The division by rhoAir is to match taur units with the ustar. It could be more tidy in the code, but at the risk of obscuring the evolution of the algorithm.

[chenhj]

Thank you very much for your prompt and detailed explanation！

Hongjing